Abstract
Abstract text here.
1 Biomark, Inc., 705 South 8th Street, Boise, Idaho, 83702, USA
2 Washington Department of Fish and Wildife, Under A Bridge, Seattle, Washington, 00000, USA
3 Mount Hood Environmental, PO Box 4282, McCall, Idaho, 83638, USA
✉ Correspondence: Richard A. Carmichael <Richard.Carmichael@merck.com>
Keywords: northern pikeminnow; Chinook salmon; predation; mark-recapture; bioenergetics
The Upper Salmon River major population group (MPG) supports eight independent, extant spring/summer Chinook Salmon Oncorhynchus tshawytscha populations including Salmon River (above Redfish Lake), Valley Creek, Yankee Fork Salmon River, East Fork Salmon River, Salmon River (mainstem below Redfish), Pahsimeroi River, Lemhi River, and North Fork Salmon River (NOAA 2017). At least five of these eight populations must meet criteria set forth by McElhany et al. (2000) and ICTRT (2007) for the MPG to be considered viable and for the recovery of the Snake River Evolutionary Significant Unit (ESU). Populations within the ESU have substantial cultural value, support downriver mainstem Snake and Columbia River commercial and subsistence fisheries, and support local fisheries and economies in years with sufficient abundance. All populations within the Upper Salmon River MPG have become depleted in recent decades. Declines in survival of juvenile Chinook Salmon have been attributed to the removal of beavers from the landscape (fur trade), mining activities, river simplification, water withdrawals, logging activities, urbanization, avian predation, proliferation of non-native species (e.g., non-native coastal rainbow trout O. mykiss irideus and brook trout Salvelinus fontinalis), warming streams and rivers, and modifications to downriver migration corridors (e.g., from hydropower projects). Moreover, the abundance of returning adults are further impacted by ocean and downriver harvests, poor ocean conditions, and changes to the spawning migration corridor. Each of these factors have contributed, to varying and unknown extents, to reduced adult escapement, the primary metric used to assess population viability. In response to the decline in Chinook Salmon abundance from the myriad human activities and associated habitat degradation, action agencies have attempted to improve juvenile survival and adult spawning conditions by investing in the rehabilitation of tributary ecosystems.
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One potentially important, but perhaps under-appreciated source of mortality on Chinook salmon is predation on emigrating juveniles by piscivorous fishes, including both native and non-native species. As an example,…
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Spring/summer Chinook Salmon in the Upper Salmon MPG are stream-type and exhibit two distinct migration tactics; downstream rearing (DSR) and natal reach rearing (NRR) (Copeland et al. 2014). The DSR migrants leave the natal spawning area as subyearlings between June and November and typically overwinter in downstream, mainstem habitats until the following spring when they emigrate to the ocean as smolts. Alternatively, NRR migrants remain in their natal spawning areas for approximately one year after emergence until emigration to the ocean as smolts. Diversity of migratory tactics provides a mechanism for coping with adverse conditions in freshwater rearing and migration environments and buffers against catastrophic events, thereby increasing population resiliency.
The Deadwater Slough is located in a reach of the Salmon River that is believed to be a historically important overwinter rearing area for DSR emigrants. Moreover, this reach is part of the migratory pathway for juvenile DSR and NRR emigrants from all eight extant populations. The slough lacks hydrological and structural features (i.e., a homogenous channel with fine substrate and little cover) that can provide essential refuge from predation. As a result, predation on juvenile Chinook salmon proximal to the Deadwater Slough has been cited as a concern for the Upper Salmon River MPG, impacting DSR migrants in the fall and NRR emigrants during the spring.
We hypothesize that increased densities of piscivorous predators in the Deadwater Slough area may explain the reduced survival (or apparent survival) observed for juvenile Chinook Salmon (Ackerman et al. 2018) and Sockeye Salmon (Axel et al. 2015). In this study, we estimated the abundance of a piscivorous fish predator population in the Deadwater Slough and their potential impacts to juvenile salmon emigrants, focusing on DSR and NRR Chinook Salmon. Our objectives for the study were four-fold:
We follow with a discussion of the various assumptions that went into the mark-recapture and bioenergetics models and assessment of impacts to adult returns and how violations of some assumptions may affect overall results and inferences from the study.
The Deadwater Slough is an approximately 1.5 kilometer section of the mainstem Salmon River located roughly 5.8 river kilometers downstream from the town of North Fork, Idaho (Figure 3). The downstream end of the slough is located at the confluence of Dump Creek and the Salmon River. Around 1897, the failure of a small mining diversion reservoir in the Dump Creek drainage resulted in an erosion event that deposited substantial amounts of sediment at the confluence of the Salmon River, thereby creating an unnaturally slow and deep section in the river, spanning approximately 30 acres and averaging 68 m width. Both northern pikeminnow Ptychocheilus oregonensis and smallmouth bass Micropterus dolomieu inhabit the slough which likely provides favorable conditions for their feeding and growth (e.g., reduced water velocity, deep channel, warmer water temperature).
Data = C/M/R and Effort
The Chapman-modified Lincoln-Peterson estimator is below, where \(M\) is the number of fish marked and returned to the population, \(n\) is the number of fish caught in the second/recapture event and \(m\) is the number of marked fish in the second sample. \[ \hat{N} = \frac{(M + 1)(n + 1)}{(m + 1)} - 1 \] The Schnabel estimator is shown below, where the \(M\), \(n\) and \(m\) are indexed by the sampling occasion, \(i\). This estimator does not have an associate standard error, but 95% confidence intervals can be calculated. \[ \hat{N} = \frac{\sum\limits_{i = 1}^k n_i M_i}{\left(\sum\limits_{i = 1}^k m_i \right) + 1} \]
Data = Gastric Lavage Data
The goal was to document predation upon juvenile Chinook salmon (or other targets), and ideally, estimate the proportion of their diet that consisted of juvenile Chinook salmon at the time of sampling?
We collected stomach contents from most captured individuals using gastric lavage (Foster (1977)) and examined contents for the presence or absence of juvenile Chinook salmon and other incidentals (e.g., steelhead, sockeye salmon) juveniles and the proportion of stomach contents containing targets versus non-targets (e.g., macroinvertebrates). Stomach contents were stored in whirl-paks, preserved with 99% isopropyl alcohol, and analyzed one week later in a controlled environment. Each sample was uniquely identified to match up with the appropriate fish record, contents were identified down to its unique composition, total weight of all content was measured in grams, and total weight of fish content, if found, was measured in grams. Throughout the sampling period a proportion of the captured individuals were sacrificed after gastric lavage to validate that the gastric lavage was successful at flushing all or most of the stomach contents from the northern pikeminnow.
Data = Temperature, Bioenergetics Inputs, Others?
Pull material from technical report
Data = SARs, Adult Escapements, Others?
Pull material from technical report
| Sampling Event | Estimator | N | SE | Lci | Uci |
|---|---|---|---|---|---|
| Fall 2019 | Chapman | 13,298 | 4,322.3 | 6,898 | 27,893 |
| Fall 2019 | Petersen | 15,105 | 5,658.3 | 7,331 | 37,569 |
| Fall 2019 | Schnabel | 18,732 |
|
10,057 | 37,851 |
| Fall 2019 | Schumacher-Eschmeyer | 20,615 |
|
14,393 | 36,313 |
| Fall 2020 | Chapman | 24,882 | 9,253.8 | 11,784 | 56,907 |
| Fall 2020 | Petersen | 29,700 | 13,170.0 | 12,727 | 91,470 |
| Fall 2020 | Schnabel | 37,556 |
|
18,698 | 82,105 |
| Fall 2020 | Schumacher-Eschmeyer | 43,279 |
|
23,061 | 351,090 |
Figure 1: Estimates of abundance of northern pikeminnow using different estimators.
Figure 2: Total catch per unit effort across entire sampling event.
We estimated the population size of Northern Pikeminnow in the Deadwater Slough to be greater than xx,xxx during the fall emigration period for DSR Chinook Salmon. That estimate translates to a density of xxx Northern Pikeminnow per 100 m or xxx per 100 m2 which is similar/more/less than estimates from elsewhere in the Columbia River (citation) where substantial Northern Pikeminnow predation impacts on salmonids have led to bounty programs aimed at reducing Northern Pikeminno abundance. The population size of Northern Pikeminnow was not directly estimated during the spring NRR Chinook salmon emigration period however, the relative abundance measured at CPUE was comparable to the fall sampling periods (update statement later). The population of Northern Pikeminnow in Deadwater Slough was estimated to consume between xx,xxx and xx,xxx juvenile Chinook Salmon during the x sampling periods and result in an estimated reduction of returning adults between xxx and x,xxx. We suggest that the habitat modifications that created the Deadwater Slough reach have resulted in favorable conditions for Northern Pikeminnow, including improved conditions for predation upon juvenile Chinook Salmon (add detail here). Therefore, predation by Northern Pikeminnow in the Deadwater Slough likely has a consequential impact on ESA-listed Chinook Salmon populations in the Upper Salmon River MPG.
Re-hash out our assumptions? What assumptions were most likely violated e.g., the closed population assumption? If we assumed an open population with varying immigration/emigration rates how might that affect our estimate? In the end, how good do we think our estimate is and is the bottom-line that we belive there still to be a shit-ton of pikeminnow even if our assumptions were violated?
Concerns with gastric lavage. We only observed juvenile Chinook salmon (and/or other fishes) in a very small number of stomach contents. However, we don’t necessarily believe that to mean that pikeminnow in Deadwater Slough rarely consume juvenile Chinook salmon. Are there other cases where gastric lavage failed to document predation? How soon after consumption do you need to take a sample? So instead, we made some assumptions about the proportion of a pikeminnows diet consisting of Chinook salmon.
What assumptions did we make during the bioenergetics assessment? And how might violations of those assumptions change our estimate of the number of juvenile Chinook salmon consumed and resulting impacts to adult returns?
Again, what assumptions did we make here and how might violations of those assumptions change our estimate of impacts to adult returns.
Although not formally assessed in this study, avian predators including Great Blue Herons Ardea herodias and Bald Eagles Haliaeetus leucocephalus are another potential source of mortality for juvenile salmon in the Deadwater Slough. The Deadwater Slough is recognized as an important bird watching and nesting area due to the riparian and backwater habitats created by the feature (Deadwater Slough - Audubon Important Bird Areas). Several piscivorous bird species have been documented using the Deadwater Slough that include the Common Mergus merganser and Hooded Lophodytes cucullatus mergansers, the Great Blue Heron, the Double-crested Cormorant Phalacrocorax auritus, and the Belted Kingfisher Megaceryle alcyon (eBird 2021). During the intial sampling event in 2019, a two-person crew walked the entire reach and surrounding and upstream areas scanning for passive integrated transponder (PIT) tags. During that informal survey, nine PIT tags were recovered near active bird nests or in an upstream anastomizing reach where herons and eagles are prevalent, suggesting that mortality may have been a result of avian predation. The PIT tag histories in PTAGIS indicate these tags were implanted into a combination of juvenile Chinook Salmon (3), Sockeye Salmon (3), and steelhead (3). Avian predation contributes a major component of the total mortality for yearling Chinook Salmon in some locations in the lower Snake River and Columbia River, particularly at hydroelectric dams and within reservoirs (Evans et al. 2012; 2016); however, we did not observe large colonies of piscivorous birds within the study area. Although there is documentation of individual Double-crested Cormorants (eBird 2021) at the Deadwater Slough, the site is not within their breeding range, rather, it is part of a migration corridor. Given the current avian species known to occupy Deadwater Slough, it is unlikely that avian predation on juvenile salmonids is comparable to elsewhere in the Columbia River basin with large piscivorous bird colonies. Nevertheless, we hypothesize that the reservoir-like conditions at the Deadwater Slough may increase the probability of avian predation on juvenile Chinook Salmon from the many piscivorous birds known to use the site. Future estimates of predation would benefit from consideration of the contribution of piscivorous avian predators.
Discuss the potential for various management actions: * Removing the Dump Creek alluvial fan and restoring flow. What would that look like? * A local bounty program? What would that look like? Have they been successful elsewhere e.g., in the Columbia River?
Deadwater is an opportunity to benefit multiple (6 or 7) local populations with a single management or restoration action.
Also, this is an anthropogenically created dead water area, which could be a candidate for restoration (removing the impoundment). Is there anything in the literature that discusses habitat preferences for pike minnow? If we speed up velocities and add some cover potentially pike minnow predation success will be lowered. Will these fish just move elsewhere?